Fix SI-7107: scala now thinks every exception is polymorphic

We need to force info of the `cls` in `parseExceptions`
because we pass `cls` to `addThrowsAnnotation` which in turn calls
`Symbol.isMonomorphicType` that relies on a symbol being initialized
to give right answers.

In the future we should just clean up implementation of
`isMonomorphicType` method to not rely on a symbol being initialized
as there's no inherent reason for that in most cases. In cases where
there's reason for that we should just force the initialization.

This patch does not come with a test-case because it's hard to reproduce
not initialized symbols in partest reliably.
Merge pull request #2117 from gkossakowski/issue/SI-7107

Fix SI-7107: scala now thinks every exception is polymorphic